Embodiments described herein relate generally to computer-automated image processing of time sequences of volume image data sets of the abdomen.
In the medical field, three-dimensional (3D) image data sets, i.e. volume image data sets, are collected by a variety of techniques—referred to as modalities in the field—including computer-assisted tomography (CT), magnetic resonance (MR), ultrasound and positron-emission-tomography (PET).
For a number of years, perfusion in abdominal organs has been measured using so-called four-dimensional (4D) dynamic contrast enhanced CT (DCE-CT). In outline, the following procedure is applied. A bolus of contrast agent is injected. Then typically between 10 and 25 CT scans are acquired at intervals of a few seconds. The relevant vasculature and organs are identified in each scan and their CT densities measured. Using a calibration curve, CT densities can be converted to contrast agent concentrations. The sequence of concentrations at each locus is plotted against time, providing a perfusion curve for the locus.
FIG. 1 shows an example perfusion curve intensity in Houndsfield Units (HU), which is proportional to perfusion, against time (T). Each point on the curve is from the same locus in the patient as indicated by the arrows leading from the perspective 2D image panels.
Early work was based on time sequences of 2D slices, while more recently the advent of multi-slice scanners has enabled the rapid capture of time sequences of 3D CT volume images. The recent introduction of 320-slice CT scanners that can acquire high-resolution CT images with up to 16 cm axial extent in a single gantry revolution has made it feasible to capture multiple sequential images of entire organs such as the kidneys or the liver while delivering a relatively small total X-ray dose.
If the patient is completely immobile both externally and internally throughout the procedure, then the loci of interest need only be identified in a single scan, and can be automatically transferred to all other scans. Clearly, this greatly reduces the interaction time needed to generate the perfusion results. In reality, respiration-induced internal motion provides a significant challenge to the image processing. Two motion-reducing protocols are known. In one, the patient is instructed to hold their breath throughout the capture sequence. However, noting that patients are often sick, elderly, or both, a significant number fail to achieve this. Instead, at some point in the capture sequence they break the breath-hold with a deep gasping breath that results in major motion (often more than 30 mm) of the abdominal organs. An alternative protocol is to ask the patient to breath regularly but as shallowly or quietly as possible. In this case, the maximum extent of abdominal organ motion is reduced, at the expense of ubiquitous albeit smaller amounts of motion. Finally, not all patients even succeed in lying still on the CT table so that whole-body motions, usually in effect small rotations about the patient's longitudinal axis, are not unknown.
One rather unsatisfactory approach is simply to identify and ignore those volume images in which large organ motion is present. Automatic volume registration provides a better approach. Automatic registration has been widely used in DCE-CT and in the related field of DCE-MRI for almost two decades and is now available from a number of manufacturers as part of their perfusion analysis capabilities.
Respiration-induced motion affects the abdomen in a rather complex fashion. Put simply, the internal organs (liver, kidneys, spleen, pancreas, etc.) tend to move approximately axially (Z), driven by the downward motion of the diaphragm. Deep breathing can cause overall axial (Z) displacements of 30 mm or more, with smaller motion in the coronal (Y) direction and, usually, least motion in the sagittal (X) axis. In general the different organs will move by different amounts. For a patient in the usual prone position, the vertebrae and rear abdominal wall remain approximately motionless. The front abdominal wall on the other hand tends to move mostly in the sagittal direction. The axial motion of the internal organs and the relatively immobile spine and abdominal wall have a boundary that is effectively discontinuous in places; the organs appear to “slide” along the inner surface of the abdominal wall. Finally, the organs are not themselves rigid but can subtly change in shape in addition to their overall gross movement.